The use of Surface Plasmon Resonance (SPR) for the detection of small soluble analytes from solution is well known (see e.g. "Advances in Biosensors--A Research Annual Vol 1. 1991" Ed. A P F Turner, Pub. Jai Press Ltd, London).
Briefly, an SPR apparatus generally comprises a light source for generating polarised light; a sensor, the outside of which is metal coated and may be contacted with a sample solution, and means for detecting the light which is internally reflected from the inner sensor surface.
In the absence of bound analyte, light is totally internally reflected at an incident angle characteristic of the refractive index (RI) of the sensor and of the sample solution. At a particular incident angle (the `SPR angle`), interaction of the metal with the evanescent wave set up by internal reflection of the polarised light causes a drop in intensity of the reflected light. This drop can be observed using the light detector.
The binding of analyte to the sensor surface, within the evanescent wave zone, alters the RI of the sensor and this perturbs the SPR angle. This perturbation can be observed using the light sensor and related to the surface concentration of analyte.
SPR detection in the literature has generally been limited to use with soluble molecular size analytes e.g. biomolecules such as proteins and nucleic acids which are specifically bound within the evanescent zone using appropriate ligands.
However, the SPR apparatus in the art to date has not been suitable for accurately detecting sample materials with both soluble and insoluble analytes therein. In particular, due to the more limited way in which (for instance) roughly spherical cells of several .mu.m diameter interact with the evanescent zone, only fairly high concentrations (e.g. 10.sup.7 -10.sup.8 /ml) have been detectable using SPR. Thus in order to detect cells, as opposed to (for instance) protein antigens, further apparatus, and hence more cost, time and experimentation, have been required. For instance cells have frequently been detected using culture techniques followed by specific detection.